Susceptor integration into reinforced thermoplastic composites

ABSTRACT

Problems associated with accurately positioning a susceptor at the bond line between two or more prefabricated, fiber reinforced, resin composite detail parts to facilitate thermoplastic welding (i.e., fusion bonding) between the detail parts. The integrated susceptor detail part is compatible with Z-pinning along the bond line for the pulloff strength enhancement associated with Z-pin reinforcement. We make the parts by (1) preparing the faying overface of the prefabricated part or prepreg preform for the overlayment of the bond line materials, (2) placing a thermoplastic film on the part or preform, (3) placing the susceptor on the film, and (4) securing the film and susceptor to the part by applying suitable heat and pressure to produce a prefabricated, consolidated detail part having an integrated susceptor in a resin rich region intended as the bond line to other parts in the final assembly.

REFERENCE TO RELATED APPLICATION

The present application claims the benefit of U.S. Provisional PatentApplication 60/019,354, filed Jun. 6, 1996.

1. Technical Field

The present invention relates to reinforced, thermoplastic compositedetail parts having a welding susceptor incorporated along at least onefaying surface and to methods for welding together assemblies from thedetail parts.

2. Background Art

The use of composites in primary structure in aerospace applications islimited today because of the relatively high cost. A significantcontribution to the total cost is the assembly cost where the precuredcomposite elements are assembled, drilled, and fastened. The necessarydesign for mechanical fastening complicates the structure, especially inthin sections, because of the need for access to both sides of the bondline.

While composites might be adhesively bonded, cocured, or welded, theseconnecting processes generally produce bonds that rely upon the resinmatrix for strength. The bond line lacks any reinforcing material tohelp with load transfer. These bonds generally have modest strength, andare susceptible to disbonding with shock impact or other "out of plane"forces affecting the assembly. Such forces often arise in environmentsprone to vibration.

1. Composite Manufacturing

Fiber-reinforced organic resin matrix composites have a highstrength-to-weight ratio (specific strength) or a highstiffness-to-weight ratio (specific stiffness) and desirable fatiguecharacteristics that make them increasingly popular as a replacement formetal in aerospace applications where weight, strength, or fatigue iscritical. Thermoplastic or thermoset organic resin composites would bemore economical with improved manufacturing processes that reduced touchlabor and forming time.

Prepregs combine continuous, woven, or chopped reinforcing fibers withan uncured matrix resin, and usually comprise fiber sheets with a thinfilm of the matrix. Sheets of prepreg generally are placed (laid-up) byhand or with fiber placement machines directly upon a tool or die havinga forming surface contoured to the desired shape of the completed partor are laid-up in a flat sheet which is then draped and formed over thetool or die to the contour of the tool. Then the resin in the prepreglay up is consolidated (i.e. pressed to remove any air, gas, or vapor)and cured (i.e., chemically converted to its final form usually throughchain-extension or fused into a single piece) in a vacuum bag process inan autoclave (i.e., a pressure oven) to complete the part.

The tools or dies for composite processing typically are formed to closedimensional tolerances. They are massive, must be heated along with theworkpiece, and must be cooled prior to removing the completed part. Thedelay caused to heat and to cool the mass of the tools addssubstantially to the overall time necessary to fabricate each part.These delays are especially significant when the manufacturing run islow rate where the dies need to be changed frequently, often afterproducing only a few parts of each kind. An autoclave has similarlimitations; it is a batch operation.

In hot press forming, the prepreg is laid-up to create a preform, whichis bagged (if necessary), and placed between matched metal tools thatinclude forming surfaces to define the internal, external, or both moldlines of the completed part. The tools and composite preform are placedwithin a press and then the tools, press, and preform are heated.

The tooling in autoclave or hot press fabrication is a significant heatsink that consumes substantial energy. Furthermore, the tooling takessignificant time to heat the composite material to its consolidationtemperature and, after curing the composite, to cool to a temperature atwhich it is safe to remove the finished composite part.

As described in U.S. Pat. No. 4,657,717 a flat composite prepreg panelwas sandwiched between two metal sheets made from a superplasticallyformable alloy, and was formed against a die having a surface preciselycontoured to the final shape of the part.

Attempts have been made to reduce composite fabrication times byactively cooling the tools after forming the composite part. Theseattempts have shortened the time necessary to produce a composite part,but the cycle time for heating and cooling remains long. Designing andmaking tools to permit their active cooling also increases their cost.

Boeing described a process for organic matrix forming and consolidationusing induction heating in U.S. Pat. No. 5,530,227. There, Boeing laidup prepregs in a flat sheet sandwiched between aluminum susceptor facesheets. The face sheets were susceptible to heating by induction andformed a retort to enclose the prepreg preform. To ensure an inertatmosphere around the preform during curing and to permit withdrawingvolatiles and outgassing during the consolidation, the face sheets arewelded around their periphery. Such welding unduly increases thepreparation time and the cost for part fabrication. It also ruined theface sheets (i.e.) prohibited their reuse which added a significant costpenalty to each part fabricated with this approach). Boeing described inU.S. Pat. No. 5,599,472 a technique that readily and reliably sealed theface sheets of the retort without the need for welding and permittedreuse of the face sheets in certain circumstances. This "bag-and-seal"technique applies to both resin composite and metal processing.

2. Processing in an Induction Press

The dies or tooling for induction processing are ceramic because aceramic is not susceptible to induction heating and, preferably, is athermal insulator (i.e., a relatively poor conductor of heat). Ceramictooling is strengthened and reinforced internally with fiberglass rodsor other appropriate reinforcements and externally with metal or otherdurable strongbacks to permit it to withstand the temperatures andpressures necessary to form, to consolidate, or otherwise to process thecomposite materials or metals. Ceramic tools cost less to fabricate thanmetal tools of comparable size and have less thermal mass than metaltooling, because they are unaffected by the induction field. Because theceramic tooling is not susceptible to induction heating, it is possibleto embed induction heating elements in the ceramic tooling and to heatthe composite or metal retort without significantly heating the tools.Thus, induction heating can reduce the time required and energy consumedto fabricate a part.

While graphite or boron fibers can be heated directly by induction, mostorganic matrix composites require a susceptor in or adjacent to thecomposite material preform to achieve the necessary heating forconsolidation or forming. The susceptor is heated inductively andtransfers its heat principally through conduction to the preform orworkpiece that, in our prior work, is sealed within the susceptorretort. Enclosed in the metal retort, the workpiece does not experiencethe oscillating magnetic field which instead is absorbed in the retortsheets. Heating is by conduction from the retort to the workpiece.

Induction focuses heating on the retort (and workpiece) and eliminateswasteful, inefficient heat sinks. Because the ceramic tools in ourinduction heating workcell do not heat to as high a temperature as themetal tooling of conventional, prior art presses, problems caused bydifferent coefficients of thermal expansion between the tools and theworkpiece are reduced. Furthermore, Boeing's induction heating press isenergy efficient because significantly higher percentages of inputenergy go to heating the workpiece than occurs with conventionalpresses. The reduced thermal mass and ability to focus the heatingenergy permits change of the operating temperature rapidly whichimproves the products produced. Finally, the shop environment is notheated as significantly from the radiation of the large thermal mass ofa conventional press. The shop is a safer and more pleasant environmentfor the press operators.

In induction heating for consolidating and forming organic matrixcomposite materials, Boeing generally places a thermoplastic organicmatrix composite preform of PEEK or ULTEM, for example, within the metalsusceptor envelope (i.e., retort). These thermoplastics have a lowconcentration of residual volatile solvents and are easy to use. Thesusceptor face sheets of the retort are inductively heated to heat thepreform. Consolidation and forming pressure consolidate and, ifapplicable, form the preform at its curing temperature. The sealedsusceptor sheets form a pressure zone in the retort in a manneranalogous to conventional vacuum bag processes for resin consolidation.The retort is placed in an induction heating press on the formingsurfaces of dies having the desired shape of the molded composite part.After the retort and preform are inductively heated to the desiredelevated temperature, differential pressure (while maintaining thevacuum in the pressure zone around the preform) across the retort whichfunctions as a diaphragm in the press forms the preform against the dieinto the desired shape of the completed composite panel.

The retort often includes three, stacked susceptor sheets sealed aroundtheir periphery to define two pressure zones. The first pressure zonesurrounds the composite panel/preform or metal workpiece and isevacuated and maintained under vacuum. The second pressure zone ispressurized (i.e., flooded with gas) at the appropriate time to helpform the composite panel or workpiece. The shared wall of the threelayer sandwich that defines the two pressure zones acts as thediaphragm.

Boeing can perform a wide range of manufacturing operations in itsinduction heating press. These operations have optimum operatingtemperatures ranging from about 350° F. (175° C.) to at least about1850° F. (1010° C.). For each operation, Boeing usually holds thetemperature relatively constant for several minutes to several hours tocomplete the operations. While temperature can be controlled bycontrolling the input power fed to the induction coil, a better andsimpler way capitalizes on the Curie temperature. Judicious selection ofthe metal or alloy in the retort's susceptor face sheets avoidsexcessive heating irrespective of the input power. With improved controland improved temperature uniformity in the workpiece, Boeing producesbetter products. The method capitalizes on the Curie temperaturephenomenon to control the absolute temperature of the workpiece and toobtain substantial thermal uniformity in the workpiece by substantiallymatching the Curie temperature of the susceptor to the desiredtemperature of the induction heating operation being performed. Thistemperature control method is explained in greater detail in our U.S.Pat. No. 5,728,309.

3. Thermoplastic Welding

Three major joining technologies exist for aerospace compositestructure: mechanical fastening; adhesive bonding; and welding. Bothmechanical fastening and adhesive bonding are costly, time consumingassembly steps that introduce excess cost even if the parts that areassembled are fabricated from components produced by an emerging, costefficient process. Mechanical fastening requires expensive holelocating, drilling, shimming, and fastener installation, while adhesivebonding often requires complicated surface pretreatments.

In contrast, thermoplastic welding, which eliminates fasteners, joinsthermoplastic composite components at high speeds with minimum touchlabor and little, if any, pretreatments. A conventional weldinginterlayer tape (compromising the susceptor and surroundingthermoplastic resin either coating the susceptor or sandwiching it) alsocan simultaneously take the place of shims required in mechanicalfastening. As such, composite welding promises to be an affordablejoining process. For "welding" a combination of thermoplastic andthermoset composite parts together, the resin that the susceptor meltsfunctions as a hot melt adhesive. If fully realized,thermoplastic-thermoset bonding will further reduce the cost ofcomposite assembly.

There is a large stake in developing a successful induction weldingprocess. Its advantages versus traditional composite joining methodsare:

reduced parts count versus fasteners

minimal surface preparation, in most cases a simple solvent wipe toremove surface contaminants

indefinite shelf life at room temperature

short process cycle time, typically measured in minutes

enhanced joint performance, especially hot/wet and fatigue

permits rapid field repair of composites or other structures.

There is little or no loss of bond strength after prolonged exposure toenvironmental influences.

U.S. Pat. No. 4,673,450 describes a method to spot weld graphite fiberreinforced PEEK composites using a pair of electrodes. After rougheningthe surfaces of the prefabricated PEEK composites in the region of thebond, Burke placed a PEEK adhesive ply along the bond line, applied apressure of about 50-100 psig through the electrodes, and heated theembedded graphite fibers by applying a voltage in the range of 20-40volts at 30-40 amps for approximately 5-10 seconds with the electrodes.Access to both sides of the assembly was required in this process whichlimited its application.

Prior art disclosing thermoplastic welding with induction heating isillustrated by U.S. Pat. Nos. 3,966,402 and 4,120,712. In these patents,conventional metallic susceptors are used and have a regular pattern ofopenings of traditional manufacture. Achieving a uniform, controllabletemperature in the bond line, which is crucial to preparing athermoplastic weld of adequate integrity to permit use of welding inaerospace primary structure, is difficult with those conventionalsusceptors.

Thermoplastic welding is a process for forming a fusion bond between twofaying thermoplastic faces of two parts. A fusion bond is created whenthe thermoplastic on the surface of the two thermoplastic compositeparts is heated to the melting or softening point and the two surfacesare brought into contact, so that the molten thermoplastic mixes. Thesurfaces are held in contact while the thermoplastic cools below thesoftening temperature.

The same process parameters apply to hot melt thermoplastic adhesivebonds between prefabricated thermoset composite bonds. Simple as thethermoplastic welding process sounds, it is difficult to performreliably and repeatably in a real factory on full-scale parts to build alarge structure such as an airplane wing box. One difficulty is heatingthe bond line properly without overheating the entire structure. It alsois difficult to achieve intimate contact of the faying surfaces of thetwo parts at the bond line during heating and cooling because of (1) thenormal imperfections in the flatness of composite parts, (2) thermalexpansion of the thermoplastic during heating to the softening ormelting temperature, (3) flow of the thermoplastic out of the bond lineunder pressure (i.e., squeeze out), and (4) contraction of thethermoplastic in the bond line during cooling.

The exponential decay of the strength of magnetic fields dictates that,in induction welding processes, the susceptible structure closest to theinduction coil will be the hottest, since it experiences the strongestfield. Therefore, it is difficult to obtain adequate heating at the bondline between two graphite or carbon fiber reinforced resin matrixcomposites relying on the susceptibility of the fibers alone as thesource of heating in the assembly. For the inner plies to be hot enoughto melt the resin, the outer plies closer to the induction coil and inthe stronger magnetic field are too hot. The matrix resin in the entirepiece of composite melts. The overheating results in porosity in theproduct, delamination, and, in some cases, destruction or denaturing ofthe resin. To avoid overheating of the outer plies and to insureadequate heating of the inner plies, we use a susceptor of significantlyhigher conductivity than the fibers to peak the heating selectively atthe bond line. An electromagnetic induction coil heats a susceptor tomelt and cure a thermoplastic resin (also sometimes referred to as anadhesive) to bond the elements of the assembly together.

The current density in the susceptor may be higher at the edges of thesusceptor than in the center because of the nonlinearity of the coil,such as occurs when using a cup core induction coil like that describedin U.S. Pat. No. 5,313,037. Overheating the edges of the assembly canresult in underheating the center, either condition leading to inferiorwelds because of non-uniform curing. It is necessary to have an open ormesh pattern in the susceptor embedded at the bond line to allow theresin to create the fusion bond between the composite elements of theassembly when the resin heats and melts.

a. Moving coil welding processes

In U.S. Pat. No. 5,500,511, Boeing described a tailored susceptor forapproaching the desired temperature uniformity. This susceptor, designedfor use with the cup coil of U.S. Pat. No. 5,313,037, relied uponcarefully controlling the geometry of openings in the susceptor (boththeir orientation and their spacing) to distribute the heat evenly. Thesusceptor had a regular array of anisotropic, diamond shaped openingswith a ratio of the length (L) to the width (W) greater than 1. Thissusceptor produced a superior weld by producing a more uniformtemperature than obtainable using a susceptor having a similar array,but one where the L/W ratio was one. Changing the length to width ratio(the aspect ratio) of the diamond-shaped openings in the susceptorproduced a large difference in the longitudinal and transverseconductivity in the susceptor, and, thereby, tailored the currentdensity within the susceptor. A tailored susceptor having openings witha length (L) to width (W) ratio of 2:1 has a longitudinal conductivityabout four times the transverse conductivity. In addition to tailoringthe shape of the openings to tailor the susceptor, Boeing altered thecurrent density in regions near the edges by increasing the foil density(i.e., the absolute amount of metal). Increasing the foil density alongthe edge of the susceptor increases the conductivity along the edge andreduces the current density and the edge heating. Boeing increased foildensity by folding the susceptor to form edge strips of double thicknessor by compressing openings near the edge of an otherwise uniformsusceptor. These susceptors were difficult to reproduce reliably. Also,their use forced careful placement and alignment to achieve the desiredeffect.

The tailored susceptor was designed to use with the cup coil of U.S.Pat. No. 5,313,037, where the magnetic field is strongest near the edgesbecause the central pole creates a null at the center. Therefore, thetailored susceptor was designed to counter the higher field at the edgesby accommodating the induced current near the edges. The highlongitudinal conductivity encouraged induced currents to flowlongitudinally.

The selvaged susceptor for thermoplastic welding which is described inU.S. Pat. No. 5,508,496 controls the current density pattern during eddycurrent heating by an induction coil to provide substantially uniformheating to a composite assembly and to insure the strength and integrityof the weld in the completed part. This susceptor is particularlydesirable for welding ribs between prior welded spars using anasymmetric induction coil (described in U.S. Pat. No. 5,444,220, whichwe incorporate by reference), because it provides a controllable area ofintense, uniform heating under the poles of the coil, a trailing regionwith essentially no heating, and a leading region with minor preheating.

Boeing achieved better performance (i.e., more uniform heating) in ribwelding by using a selvaged susceptor having edge strips withoutopenings. The resulting susceptor, then, has a center portion with aregular pattern of openings and solid foil edges, referred to as selvageedge strips. The susceptor is embedded in a thermoplastic resin to makea susceptor/resin tape that is easy to handle and to use in preformingthe composite pieces prior to welding. Also, with a selvaged susceptor,the impedance of the central portion should be anisotropic with a lowertransverse impedance than the longitudinal impedance. Here, the L/Wratio of diamond shaped openings should be less than or equal to one.With this selvaged susceptor in the region immediately under theasymmetric induction work coil, current flows across the susceptor tothe edges where the current density is lowest and the conductivity,highest.

Generally, the selvaged susceptor is somewhat wider than normal so thatthe selvage edge strips are not in the bond line. Boeing sometimesremoves the selvage edge strips after forming the weld, leaving only aperforated susceptor foil in the weld. This foil has a relatively highopen area fraction.

Another difficulty remaining in perfecting the thermoplastic weldingprocess for producing large scale aerospace structures in a productionenvironment involved control of the surface contact of the fayingsurfaces of the two parts to be welded together. The timing, intensity,and schedule of heat application is controlled so the material at thefaying surfaces is brought to and maintained within the propertemperature range for the requisite amount of time for an adequate bondto form. Intimate contact is maintained while the melted or softenedmaterial hardens in its bonded condition.

Large scale parts such as wing spars and ribs, and the wing skins thatare bonded to the spars and ribs, are typically on the order of 20-30feet long at present, and potentially as much as 100 feet in length whenthe process is perfected for commercial transport aircraft. Parts ofthis magnitude are difficult to produce with perfect flatness. Instead,the typical part will have various combinations of surface deviationsfrom perfect flatness, including large scale waviness in the directionof the major length dimension, twist about the longitudinal axis,dishing or sagging of "I" beam flanges, and small scale surface defectssuch as asperities and depressions. These irregularities interfere withfull surface area contact between the faying surfaces of the two partsand actually result in surface contact only at a few "high points"across the intended bond line. Applying pressure to the parts to forcethe faying surfaces into contact achieves additional surface contact,but full intimate contact is difficult or impossible to achieve in thisway. Applying heat to the interface by electrically heating thesusceptor in connection with pressure on the parts tends to flatten theirregularities further, but the time needed to achieve full intimatecontact with the use of heat and pressure is excessive, can result indeformation of the top part, and tends to raise the overall temperatureof the "I" beam flanges to the softening point, so they begin to yieldor sag under the application of the pressure needed to achieve a goodbond.

Boeing's multipass thermoplastic welding process described in U.S. Pat.No. 5,486,684 (which we incorporate by reference) enables a moving coilwelding process to produce continuous or nearly continuous fusion bondsover the full area of the bond line. The result is high strength weldsproduced reliably, repeatably, and with consistent quality. This processproduces improved low cost, high strength composite assemblies of largescale parts fusion bonded together with consistent quality. It uses aschedule of heat application that maintains the overall temperature ofthe structure within the limit in which it retains its high strength.Therefore, it does not require internal tooling to support the structureagainst sagging which otherwise could occur when the bond line is heatedabove the high strength temperature limit. The process also producesnearly complete bond line area fusion on standard production compositeparts having the usual surface imperfections and deviations from perfectflatness. The multipass welding process eliminates fasteners and theexpense of drilling holes, inspecting the holes and the fasteners,inspecting the fasteners after installation, sealing between the partsand around the fastener and the holes; reduces mismatch of materials;and eliminates arcing from the fasteners.

In the multipass process, an induction heating work coil is passedmultiple times over a bond line while applying pressure in the region ofthe coil to the components to be welded, and maintaining the pressureuntil the resin hardens. The resin at the bond line is heated to thesoftening or melting temperature with each pass of the induction workcoil and pressure is exerted to flow the softened/melted resin in thebond line and to reduce the thickness of the bond line. The pressureimproves the intimacy of the faying surface contact with each pass toimprove continuity of the bond. The total time at the softened or meltedcondition of the thermoplastic in the faying surfaces is sufficient toattain deep interdiffusion of the polymer chains in the materials of thetwo faying surfaces throughout the entire length and area of the bondline. The process produces a bond line of improved strength andintegrity in the completed part. The total time of the faying surfacesat the melting temperature is divided into separate time segments whichallows time for the heat in the interface to dissipate without raisingthe temperature of the entire structure to the degree at which it losesits strength and begins to sag. The desired shape and size of the finalassembly is maintained.

A structural susceptor includes fiber reinforcement within the weldresin to alleviate residual tensile strain otherwise present in anunreinforced weld. This susceptor includes alternating layers of thinfilm thermoplastic resin sheets and fiber reinforcement (usually wovenfiberglass fiber) sandwiching the conventional metal susceptor that isembedded in the resin. While the number of total plies in thisstructural susceptor is usually not critical, Boeing prefers to use atleast two plies of fiber reinforcement on each side of the susceptor.This structural susceptor is described in greater detail in U.S. patentapplication Ser. No. 08/471,625, which we incorporate by reference.

The structural susceptor permits gap filling between the weldedcomposite laminates which tailors the thickness (number of plies) in thestructural susceptor to fill the gaps, thereby eliminating costlyprofilometry of the faying surfaces and the inherent associated problemof resin depletion at the faying surfaces caused by machining thesurfaces to have complementary contours. Standard manufacturingtolerances produce gaps as large as 0.120 inch, which are too wide tocreate a quality weld using the conventional susceptors.

It is easy to tailor the thickness of the structural susceptor to matchthe measured gap by scoring through the appropriate number of plies ofresin and fiber reinforcement and peeling them off. In doing so, a resinrich layer will be on both faying surfaces and this layer should insurebetter performance from the weld.

b. Fixed coil induction welding

Thermoplastic welding using Boeing's induction heating workcell differsfrom the moving coil processes because of the coil design and resultingmagnetic field. The fixed coil workcell presents promise for welding atfaster cycle times than the moving coil processes because it can heatmultiple susceptors simultaneously. The fixed coil can reduce operationsto minutes where the moving coil takes hours. The keys to the process,however, are achieving controllable temperatures at the bond line in areliable and reproducible process that assures quality welds of highbond strength. The fixed coil induces currents to flow in the susceptordifferently from the moving coils and covers a larger area.Nevertheless, proper processing parameters permit welding with theinduction heating workcell using a susceptor at the bond line.

Another advantage with the fixed coil process is that welding can occurusing the same tooling and processing equipment used to consolidate theskin, thereby greatly reducing tooling costs. Finally, the fixed coilheats the entire bond line at one time to eliminate the need for shimsthat are currently used with the moving coil. To control the temperatureand to protect against overheating, "smart" susceptors as a retort or asthe bond line susceptor material or both are used.

The need for a susceptor in the bond line poses many obstacles to thepreparation of quality parts. The metal which is used because of itshigh susceptibility differs markedly in physical properties from theresin or fiber reinforcement so dealing with it becomes a significantissue. The reinforced susceptor of U.S. patent application Ser. No.08/469,029 (which we also incorporate by reference) overcomes problemswith conventional susceptors by including the delicate metal foils(0.10-0.20 inch wide×0.005-0.010 inch thick; preferably 0.10×0.007 inch)in tandem with the warp fibers of the woven reinforcement fabric. Theweave fibers hold the foils in place longitudinally in the fabric inelectrical isolation from each other yet substantially covering theentire width of the weld surface while still having adequate space forthe flow and fusion of the thermoplastic resin. Furthermore, in the bondline, the resin can contact, wet, and bond with the reinforcing fiberrather than being presented with the resinphilic metal of theconventional systems. There will be a resin-fiber interface with onlyshort runs of a resin-metal interface. The short runs are the length ofthe diameter of two weave fibers plus the spatial gap between the weavefibers, which is quite small. Thus, the metal is shielded within thefabric and a better bond results. In this woven arrangement to foil canassume readily the contour of the reinforcement. Finally, thearrangement permits efficient heat transfer from the foil to the resinin the spatial region where the bond will focus.

The strength and durability of adhesive bonds or thermoplastic weldsconnecting composite structure is improved, however, by adding Z-pinmechanical reinforcement to the bond line.

4. Z-Pin Reinforcement

First, some general discussion about the benefits of Z-pins.

Composite sandwich structures having resin matrix skins or face sheetsadhered to a honeycomb or foam core are used in aerospace, automotive,and marine applications for primary and secondary structure. The facesheets typically are reinforced organic matrix resin composites madefrom fiberglass, carbon, ceramic, or graphite fibers reinforcing athermosetting or thermoplastic matrix resin. The face sheets carry theapplied loads, and the core transfers the load from one face sheet tothe other or absorbs a portion of the applied load. In either case, itis important that all layers of the structure remain rigidly connectedto one another. Noise suppression sandwich structure or sandwichstructures for other applications are described in U.S. Pat. No.5,445,861, which we also incorporate by reference.

Keeping the face sheets adhered to the foam is problematic. The mostcommon source of delamination stems from a relatively weak adhesive bondthat forms between the face sheets and the foam core. That is, pulloffstrength of the face sheets in shear is low. Efforts to strengthen thebond have generally focused on improving the adhesive, but those effortshave had limited success.

Delamination can arise from differences in the coefficient of thermalexpansion (CTE) of the different material layers. As a result, astemperatures oscillate, the face sheet or foam may expand or contractmore quickly than its adjoining layer. In addition to causing layerseparation, CTE differences can significantly distort the shape of astructure, making it difficult to maintain overall dimensionalstability. Conventional sandwich structure optimizes the thickness of astructure to meet the weight and/or space limitations of its proposedapplication. Sandwich structures are desirable because they are usuallylighter than solid metal or composite counterparts, but they may beundesirable if they must be larger or thicker to achieve the samestructural performance. Providing pass-throughs (i.e., holes), which isrelatively easy in a solid metal structure by simply cutting holes inthe desired locations, is more difficult in a composite sandwichstructure because holes may significantly reduce the load carryingcapability of the overall structure.

Foster-Miller has been active in basic Z-pin research. U.S. Pat. No.5,186,776 describes a technique for placing Z-pins in compositelaminates involves heating and softening the laminates with ultrasonicenergy with a pin insertion tool which penetrates the laminate, movingfibers in the laminate aside. The pins are inserted either when theinsertion tool is withdrawn or through a barrel in the tool prior to itsbeing withdrawn. Cooling yields a pin-reinforced composite. U.S. Pat.No. 4,808,461 describes a structure for localized reinforcement ofcomposite structure including a body of thermally decomposable materialthat has substantially opposed surfaces, a plurality of Z-pinreinforcing elements captured in the body and arranged generallyperpendicular to one body surface. A pressure plate (i.e., a caul plate)on the other opposed body surface drives the Z-pins into the compositestructure at the same time the body is heated under pressure anddecomposes. We incorporate U.S. Pat. Nos. 4,808,461 and 5,186,776 byreference.

A need exists for a method to form a sandwich structure that (1) resistsdistortion and separation between layers, in particular, separation ofthe face sheets from the core; (2) maintains high structural integrity;(3) resists crack propagation; and (4) easily accommodates the removalof portions of core, as required by specific applications. The methodshould allow the structure to be easily manufactured and formed into avariety of shapes. In U.S. patent application Ser. No. 08/582,297 (whichwe incorporate by reference), Childress described such a method offorming a pin-reinforced foam core sandwich structure. Face sheets ofuncured fiber-reinforced resin (i.e., prepreg or B-stage thermoset) areplaced on opposite sides of a foam core. The core has at least onecompressible sublayer and contains a plurality of Z-pins spanning thefoam between the face sheets. Childress inserted the Z-pins into theface sheets during autoclave curing of the face sheet resin when acompressible sublayer is crushed and the back pressure applied troughthe caul plate or other suitable means drives the Z-pins into one orboth of the face sheets to form a pin-reinforced foam core sandwichstructure. By removing some of the foam core by dissolving, eroding,melting, drilling, or the like to leave a gap between the face sheets,he produced his corresponding column core structure.

The foam core generally is itself a sandwich that includes a highdensity foam sublayer, and at least one low density, compressible orcrushable foam sublayer. The preferred arrangement includes a first andsecond low density foam sublayer sandwiching the high density sublayer.The Z-pins are placed throughout the foam core in a regular array normalto the surface or slightly off-normal at an areal density of about 0.375to 1.50% or higher, as appropriate, extending from the outer surface ofthe first low density foam sublayer through to the outer surface of thesecond low density foam sublayer. Expressed another way with respect tothe arrangement of the pins, there are 200 or more pin/inch².Preferably, the foam sublayers are polyimide or polystyrene, the Z-pinsare stainless steel or graphite, and the face sheets arefiber-reinforced, partially cured or precured thermosetting orthermoplastic resin composites.

In U.S. Pat. No. 5,589,016, Hoopingarner et al. describe a honeycombcore composite sandwich panel having a surrounding border element (i.e.,a "closeout") made of rigid foam board. The two planar faces of therigid foam board are embossed or scored with a scoring pattern ofindentations usually in the form of interlinked equilateral triangles.The scoring is sufficiently deep numerous to provide escape paths forvolatiles generated inside the panel during curing and bonding of theresin in the face sheets to the honeycomb core and peripheral foam. Thescoring prevents the development of excessive pressure between the facesheets in the honeycomb core that otherwise would interfere with thebonding. We incorporate this application by reference.

Rorabaugh and Falcone discovered two ways to increase the pulloffstrength in foam core sandwich structure. First, they form resin filletsaround the fiber/resin interfaces at the contact faces of the foam coreby dimpling the foam to create a fillet pocket prior to resin flowduring curing. Second, they arrange the pins in an ordered fashion suchas a tetrahedral configuration or a hat section configuration. Intetrahedral or hat section configurations, the pins not only provide atie between the two skins but they also provide miniature structuralsupport suited better for load transfer than normal or random off-normal(interlaced) or less ordered pin configurations. With ordering of thepins, they produce anisotropic properties. More details concerning theirZ-pin improvements are available in their U.S. patent application Ser.No. 08/628,879, which we incorporate by reference.

In U.S. patent application Ser. No. 08/658,927, Childress introducedZ-pin mechanical reinforcement to the bond line of two or more compositeelements by prefabricating cured composite elements that includeprotruding Z-pins (or stubble) along the element face that will contactthe bond line. The stubble is formed by including peel plys on this faceduring pin insertion using, for example, the process described in U.S.patent application Ser. No. 08/582,297. When connecting the element toother composite structure, Childress removed the peel plys to expose thestubble. Then, he assembled the several elements in the completedassembly to define the bond line. The problem with this Childress methodis that it introduces the Z-pins to the detail parts which forcesmodification of their manufacturing processes and tools. We make acorresponding structure having a thermoplastic welding susceptorintegrated into the faying surface of the detail part in the region ofthe bond line.

Pannell discovered a method to achieve Z-pin reinforcement usingordinary detail parts. Described in U.S. patent application Ser. No.08/660,060 (which we incorporate by reference), Pannell use precuredZ-pin strips to produce the reinforced joining of prefabricatedcomposite detail parts in adhesive bonding, cocuring, or thermoplasticwelding processes. The strips have pin stubble projecting on opposedfaces. The strips eliminate the need to incorporate the stubble into thedetail parts, which would be difficult with manufacturing operationslike resin transfer molding (RTM) or fiber placement. The strips arecompatible with all major joining processes, are easy to manufacture,are easy to store, and have lasting shelf-life. They can be used withour integrated susceptor detail parts.

Pannell's strips are easy to prepare using a press that includes asilicone layup surface instead of the hard surfaces normally used in pininsertion operations in the prior art. The silicone allows controlledpenetration of the pins. Once the strips are formed, because the resinis cured, the strips are easy to store without refrigeration or otherprotection until used in combination with conventional detail parts tofabricate joined composite assemblies.

The thin, flexible strips simplify the placement of the bond and allowthe bond line location to be determined in a determinant assemblyoperation at the time the detail parts are arranged in the assembly jig.The strip bends to assume the shape of the bond line and can fit acomplex curvature. As such, the strips eliminate the need for preciseplacement of pins in the already expensive, precision detail parts. Theability to define the bond line at the time of assembly using precuredparts also greatly simplifies the assembly process over competingprocesses that require the use of uncured parts.

Following assembly, Childress and Pannell complete the bonding, cocuringor welding using conventional joining techniques. If the stubble isbacked by a foam core sandwich structure of the type Childress describedin U.S. patent application Ser. No. 08/582,297, the connecting operationmight compress or decompose a low density sublayer in the foam to drivethe Z-pins deeper into the contacted element along the bond line.

Assemblies having Z-pin mechanical reinforcement are better able towithstand impact shock without peel failure.

The pin insertion processes that Foster-Miller uses or that Childress orPannell suggest for their research efforts into Z-pin reinforcedcomposites pose two important problems for production scale up. First,by having the foam almost directly on the part separated by only softthin materials, when the pins penetrate these layers, the resin flowsthrough the holes and saturates the foam and cloth. When you try to peelthe foam off, or rather if you can peel the foam off without a secondarymachining process, many pins adhere to the breather/foam layers and arepulled out of the laminate. Pulling the pins out creates a loss ofcontrol as to the arrangement of pins left in the part, and to theheight of each pin.

Second, the foam functions as the guide bushing and is supposed tointroduce the pins perpendicular to the plies in the laminate. However,the foam is soft. When the pins hit a fiber in the panel, they take thepath of least resistance and go around the fiber. Since the foam offerslittle support, the pins move instead of the fibers, so they end upbeing pushed into the panel at various angles. Loss of control occurs asto pin orientation and insertion depth.

Accurately positioning the susceptor at the bond line is difficult,especially for bond lines between complexly curved parts. Therefore, ina fashion analogous to Childress's adding Z-pins to the detail partsalong the faying surfaces, we have discovered how to include a susceptoron this surface with or without Z-pins. Alternatively, we can make aPannell precured strip having a susceptor mesh integrated on one or bothof the opposed surfaces. With our integrated susceptor detail parts, wesimplify the assembly of complex composite assemblies because thesusceptor is carried with one or more of the detail parts.

We can make our detail parts with exposed Z-pins like the Childressdetail parts using Avila's pin insertion machine. That machine isdescribed in greater detail in U.S. patent application Ser. No.08/657,859, which we incorporate by reference.

Accurately positioning the susceptor at the bond line remains a problemespecially if the bond line forms a complex curvature. By integratingthe susceptor into the detail, we fix its relationship to one of thedetails and know precisely where it will be located relative to theinduction coil.

SUMMARY OF THE INVENTION

We incorporate a thermoplastic welding susceptor into a detail part orprecured strip for alignment along the faying surfaces defining the bondline to simplify alignment of parts prior to welding. With the susceptorincorporated into the detail part, assembly (especially of complexlycurved assemblies) avoids tricky and difficult positioning of aseparate, conventional susceptor.

We incorporate the susceptor into the detail part by:

(a) applying a sheet of thermoplastic resin on a prefabricated part orprepreg layup to make the surface resin rich in the location of theintended bond line;

(b) overlaying a susceptor on the thermoplastic sheet; and

(c) curing the assembly under suitable heat and pressure to form thedetail with the desired, integral bond line susceptor in a resin richzone.

RTM manufacture is possible for making these integrated susceptor detailparts.

We prepare welded or bonded composite assemblies using the integratedsusceptor detail parts. The integrated susceptors are compatible withZ-pin reinforcement in the weld and with moving coil or fixed coilheating processes or even with resistance heating processes.

Integrating the susceptor onto the faying surface of the detail partfixes its location spatially. Doing so, simplifies the welding ofcomplex curvatures, because the susceptor is accurately secured at apredetermined location as a feature of the detail part. With thepart-to-part location of the susceptor identified, it is easier to planfor Z-pin reinforcement in the weld and to automate the welding process.Not only are there fewer parts, but alignments greatly simplified, sincethe relative position of the susceptor to one detail part is fixed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of Boeing's induction heating workcell.

FIG. 2 is a schematic cross-sectional view of the induction heatingworkcell of FIG. 1.

FIG. 3 is a schematic cross-sectional view of the induction heatingworkcell adapted for thermoplastic welding of a wingskin/spar assembly.

FIG. 4 is another schematic cross-sectional view of the workcell of FIG.3 rotated 90° from the view in FIG. 3.

FIG. 5 is a schematic sectional view of a reinforced susceptor.

FIG. 6 is a plan view of a multistrip susceptor that is weavable intothe fabric to form the reinforced susceptor.

FIG. 7 is a schematic isometric showing the method for forming acomposite detail part having Z-pin reinforcement stubble along a bondline.

FIG. 8 is a schematic isometric showing layup of a stubbled I-beam sparin contact with an uncured skin panel.

FIG. 9 is a schematic sectional view of the spar/panel assembly of FIG.8 illustrating the reinforced bond line.

FIG. 10 is another schematic isometric illustrating layup of a stubbledspar to a cured skin with an intermediate padup layer along the bondline.

FIG. 11 is a schematic sectional view illustrating the bond line of anassembly made in accordance with FIG. 10.

FIG. 12 is another schematic isometric illustrating layup of anotherstubbled spar to a cured skin.

FIG. 13 is a schematic sectional view illustrating the bond line of anassembly made in accordance with FIG. 12, showing the sandwich structureon the skin permitting deeper penetration of the Z-pin reinforcementinto the spar.

FIGS. 14-16 illustrate the process for making a stubbled compositedetail part.

FIG. 17 is a chart showing the improvement in compression after impactstrength for Z-pin reinforced structure.

FIG. 18 is another chart showing the improvement in T-stiffener pull-offstrength resulting from Z-pin reinforcement.

FIG. 19 is a schematic showing a padup strip that incorporates asusceptor.

FIG. 20 is a detail showing the mesh of the susceptor of FIG. 19.

FIG. 21 is a schematic sectional view of Avila's pin insertion tool.

FIG. 22 is a schematic isometric of a cure tool insert used in theinsertion tool of FIG. 21.

FIG. 23 is a schematic isometric of a pin interfacing and registeringwith a hole in the cure tool insert of FIG. 22.

FIG. 24 is a schematic exploded view of a detail part of the presentinvention having an integrated susceptor along the bond line surface.

FIG. 25 is schematic isometric, partially cutaway, showing a transverseflux induction head that we used to prepare lap shear welded testarticles.

FIG. 26 is a schematic elevation showing one typical welding operationfor joining a skin and spar.

FIG. 27 is a schematic isometric showing an interconnecting spar/skinassembly having welded joints in accordance with the processes of thepresent invention.

FIG. 28 is a schematic assembly view of the structure of FIG. 27.

FIG. 29 is a graph illustrating the increase in strength for the weldobtained with a post-weld anneal to control the rate of cooling in thebond line.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

First, some discussion about thermoplastic welding using Boeing'sinduction heating press before focusing on the integrated susceptordetail part of the present invention. We can use this press both tomanufacture the detail-parts or to join such parts into weldedassemblies.

1. The Induction Heating Press

In FIG. 1, an induction heating workcell 10 includes tools or dies 20and 22 mounted within an upper 24 and a lower 26 strongback. Thestrongbacks are each threaded onto four threaded column supports orjackscrews 28 or they float free on the columns and are fixed with nuts.We can turn the jackscrews to move one strongback relative to the other.The strongbacks 24 and 26 provide a rigid, flat backing surface for theupper and lower dies 20 and 22 to prevent the dies from bending andcracking during manufacturing operations. Preferably, the strongbackshold the dies to a surface tolerance of ±0.003 in/ft² of the formingsurface. Such tolerances are desirable to achieve proper parttolerances. The strongbacks may be steel, aluminum, or any othermaterial capable of handling the loads present during forming orconsolidation, but we prefer materials that are nonmagnetic to avoid anydistortion to the magnetic field that our induction coils produce. Insome circumstances, the dies may be strong enough themselves thatstrongbacks are unnecessary. The strongbacks transfer pressure inputthrough the columns evenly to the dies.

The dies 20 and 22 are usually ceramic and are reinforced with aplurality of fiberglass rods 32 that are held with bolts 74 and thatextend both longitudinally and transversely in a grid through each die.Each die usually is framed with phenolic reinforcement 72 as well tomaintain a compressive load on the die. Each die may be attached to itsstrongback by any suitable fastening device such as bolting or clamping.In the preferred embodiment, both dies are mounted on support plates 76which are held in place on the respective strongbacks through the use ofclamping bars 77. The clamping bars 77 extend around the periphery ofthe support plates 76 and are bolted to the respective strongbacksthrough the use of fasteners (not shown).

The dies should not be susceptible to inductive heating so that heatingis localized in the retort rather than distributed in the press as well.The ceramic has a low coefficient of thermal expansion, good thermalshock resistance, and relatively high compression strength, such as acastable fused silica ceramic.

Boeing embeds portions of an induction coil 35 in the dies. In theillustrated embodiment, we use four separate induction segments overliethe top and bottom of the workpiece, but the number can vary, as shownin FIG. 3, and the segments can surround all sides of the workpiece. Thesegments shown in FIG. 3 are formed from a straight tubing section 36that extends along the length of each die and a flexible coil connector38 that joins the straight tubing sections 36 in the upper die 20 to thecorresponding straight tubing section in the lower die 22. Otherarrangements might be used with the intent to create a known,controllable magnetic field suitable for the welding operation.Connectors 40 located at the ends of the induction coil 35 connect theinduction coil 35 to an external power source or coil driver 50 and to acoolant source. While illustrated as a circular tube, the cross-sectionis arbitrary. We often use rectangular channels for the inductionsegments.

Cavities 42 and 44 in the respective dies hold tool inserts 46 and 48.The upper tool insert 46 in some applications has a contoured formingsurface 58 that has a shape corresponding to the desired shape of theouter mold line surface of the completed composite. The lower toolinsert determines the inner mold line. The tool inserts also should notbe susceptible to inductive heating, preferably being formed of acastable ceramic. In some cases, both the dies and the tool inserts canbe made from a matrix resin rather than from a ceramic. Using a resin,however, limits use of the tooling to low temperature operations, suchas forming or consolidating certain organic matrix composites. Boeingprefers ceramic tooling which provides the greatest flexibility andversatility for the induction heating workcell.

While the forming surfaces can be an integral part of the dies, weprefer the separate die and tool insert configuration shown in FIG. 2because changing tool inserts to make different parts is easier andquicker (because they are significantly smaller) and the overall toolingcosts are reduced.

Each die surrounds and supports the respective tool insert and holds thestraight sections 36 of the induction coil in proper position inrelationship to the tool insert 46 or 48. In the preferred embodiment,the interior 70 of the dies is formed of a castable phenolic or ceramicand the exterior sides from precast composite phenolic resin blocks 72.In some applications, with chopped fibers or nonwoven or wovenreinforcing mats reinforce the phenolic or ceramic.

FIG. 2 shows a retort 60 between the tool inserts 46 and 48. The retort60 includes an organic matrix composite panel or metal workpiece andsandwiching susceptor face sheets. The retort is heated to a forming orconsolidation temperature by energizing the coil 35. In the case of acomposite panel, when the panel reaches the consolidation temperature atwhich the matrix resin flows, we apply gas pressure to the outersurfaces of the retort by pressure sources 52 and 54. Pressure source 52applies pressure to the upper surface of the retort 60 through a conduit62 that passes through the upper die 20 and upper tool insert 46, whilepressure source 54 applies a pressure to the lower surface of the retort60 through a conduit 64 that passes through the lower die 22 and lowertool insert 48. The pressure applied to the retort 60 is maintaineduntil the retort has formed to the contour of the forming surface 58 andthe matrix resin has consolidated. The pressure sources 52 and 54generally apply a differential pressure to the retort 60. We do not usea retort in the present invention.

An alternating oscillating electrical current in the induction coil 35produces a time varying magnetic field that heats the susceptor sheetsof the retort via eddy current heating. The frequency at which the coildriver 50 drives the coils 35 depends upon the nature of the retort 60.We power the coil with up to about 400 kW at frequencies of betweenabout 3-10 kHz. Current penetration of copper at 3 kHz is approximately0.06 inches (1.5 mm), while penetration at 10 kHz is approximately 0.03inches (0.75 mm).

The tool inserts and dies are usually substantially thermally insulatingand trap and contain heat within the retort. Since the dies and toolinserts are not inductively heated and act as insulators to maintainheat within the retort, the present invention requires far less energyto achieve the desired operating temperature than conventional autoclaveor resistive hot press methods where the metal tooling is a massive heatsink.

The operations using Boeing's fixed coil workcell are faster than priorart operations because it does not heat the large thermal mass of eitherthe dies or tool inserts. The retort is heated, the tool is not. Thus,the necessary processing temperature is achieved more rapidly. Inaddition, the highly conductive materials in the retort provide rapidheat transfer to the workpiece. When the driver 50 is de-energized, thedies and the retort cool rapidly to a temperature at which we can removethe retort from the workcell, saving time and energy over conventionalsystems. Coolant flowing through the coil tubes functions as an activeheat exchanger to transfer heat out of the workpiece, retort, and dies.In addition, the thermal cycle is not as limited by the heating andcooling cycle of the equipment and tools allowing better tailoring ofthe thermocycle better to the process for which we are using theinduction heating workcell.

A more complete description of the press is provided in copending U.S.Pat. Nos. 5,723,849 and 5,641,422, which we incorporate by reference.

2. Thermoplastic Welding

As shown in FIGS. 3 & 4, Boeing has made several changes to itsinduction heating workcell to adapt it to perform bond linethermoplastic welding. First, because the assemblies of primary interestare wingskin/spar combinations and because the parts in thesecombinations are prefabricated so that the welding operation need onlyfocus upon melting the thermoplastic while applying modest pressure tofacilitate the fusion, Boeing creates a cavity within its dieset tocontain the wingskin/spar combinations. The cavity is substantially acube or similar rectangular solid with canted edges and has majorsurfaces (i.e., the top and bottom) complementary to the contour of thewing assembly. The induction coil segments extend longitudinally in thesame direction as the spars and underlie the major and canted surfacesas shown in FIG. 3. The skins 100 and spars 105 are assembled in thecenter of the cavity sandwiched between, optionally, silicone rubberpressure pads 110 that assure substantially uniform pressuredistribution over the wingskin surfaces irrespective of surface featuresor irregularities. A susceptor tape 115 is positioned along the bondline between the wingskin 100 and the spar caps. By a "susceptor tape"we mean a metal ribbon embedded in thermoplastic resin, a structuralsusceptor as described in U.S. Pat. No. 5,717,191 having theresin-embedded ribbon sandwiched with alternating plies of thermoplasticfilm and fiber reinforcement to alleviate residual tensile strain in theweld and to simplify gap filling while ensuring a resin rich, qualityweld, or a reinforced susceptor as described in U.S. Pat. No. 5,723,849.The metal ribbon may be copper, a nickel-cobalt alloy, a nickel-ironalloy, a cobalt-iron alloy, or any other suitable "smart" susceptor fromthe alternatives discussed in U.S. Pat. No. 5,728,309. The susceptormight be narrow metal strips about 0.10-0.20 inch wide held inside-by-side array with the thermoplastic resin or woven with carbonfibers or other reinforcement. The induction coil induces eddy currentsthat run longitudinally. Therefore, the susceptor should have a lowerlongitudinal impedance to promote longitudinal current flow. We mightuse a modified, selvaged susceptor (see U.S. Pat. No. 5,508,496) havingsolid copper bands alternating with mesh sections with the solid bandsin the bond line rather than falling outside it, since they are theprimary current carriers. We would integrate the susceptor tape into oneface of the detail parts forming the assembly, when using the process ofthe present invention.

A "susceptor tape," however, still suffers from a relatively low bondstrength because the metal susceptor is asked to function as theequivalent of a reinforcing fiber. The matrix resin, however, does notwet with the metal as well as it does with the reinforcing fibers andthe metal does not have the strength commonly available with the fibers.Therefore, a reinforced susceptor promises improved bond strength.

The need for a susceptor in the bond line poses many obstacles to thepreparation of quality parts. The metal which is used because of itshigh susceptibility differs markedly in physical properties from theresin or fiber reinforcement so dealing with it becomes a significantissue. The reinforced susceptor (FIG. 5) overcomes problems withconventional susceptors by including the delicate metal foils 200(0.10-0.20 inch wide×0.005-0.010 inch thick; preferably 0.10×0.007 inch)in tandem with the warp fibers 205 of the woven reinforcement fabric.The weave fibers 210 hold the warp fibers 205 and foils 200 in placelongitudinally in the fabric in electrical isolation from each other yetsubstantially covering the entire width of the weld surface while stillhaving adequate space for the flow and fusion of the thermoplasticresin. Furthermore, in the bond line, the resin can contact, wet, andbond with the reinforcing fiber rather than being presented with theresinphilic metal of the conventional systems. There will be aresin-fiber interface with only short runs of a resin-metal interface.The short runs are the length of the diameter of two weave fibers plusthe spatial gap between the weave fibers, which is quite small. Thus,the metal is shielded within the fabric and a better bond results. Inthis woven arrangement to foil can assume readily the contour of thereinforcement. Finally, the arrangement permits efficient heat transferfrom the foil to the resin in the spatial region where the bond willform. The reinforced susceptor might be an analog of the structural,selvaged, or tailored susceptors of one other application (i.e. a tapeencased in resin and placed along the bond line) or may be fabricated aspart of the facing plys of the prefabricated composites that abut alongthe bond line.

The foil in the susceptor may be in the form of a multistrip susceptor112 as shown in FIG. 6. The foil includes two or more parallel strips114 that extend the full length of the strip. The foil is usually about0.007 inch thick and each strip is about 0.10-0.20 inch wide. The stripsare separated by gaps of comparable width or slightly wider dimensionwhich we etch or ablate from a solid foil. Along the length of thesusceptor, we periodically use transverse spacer strips 116 to span thegap and to retain the carrier strips 114 apart. While shown as fourstrips wide, the foil can be virtually any width. It can be abouttwo-four inches wide to match the spar cap width or might even be thefull width of sheets of the composite prepreg used to form the skins.Dimensions given are typical and could be varied.

As shown in FIG. 4, the susceptors for the top and bottom are connectedtogether into a loop circuit with jumpers 115 at the ends of the spars105. The jumpers 115 allow the current which the magnetic field inducesto flow around the assembly to generate heat in the bond lines.

With the wingskin/spar combination assembled on the pressure pads in thecavity, we close the dies and energize the coil 35 using a frequency ofabout 3-10 kHz to produce about 400 kW. This energy produces anoscillating magnetic field around the assembly (which preferably isaligned with the central axis of the coil) that rapidly heats thesusceptors to the desired welding temperature. We prefer to use a"smart" susceptor-made from a nickel-iron alloy as discussed in U.S.Pat. No. 5,728,309, which will assure that we do not overheat the bondline as well as assuring a substantially uniform temperature in the bondline during the fusion period when the thermoplastic resin is melted. Asshown in FIG. 3, we simultaneously make the six welds (one weld on eachspar cap of the three spars), which greatly reduces processing time. Thewelding process is quite fast taking about 25-30 minutes includingheating to the melt, holding the temperature during the weld fusion, andcooling. Throughout the process, we maintain a pressure of about 30-50psig along the bond line. The weight of the assembly may make thepressure slightly higher on the bottom than the top but this pressuredifference should be insignificant to the quality of the weld and theperformance of the completed part.

The welding process might cause the combination to sag when the bondline reaches the melt temperature where the flow needed for focusing thefusion bond occurs. Therefore, it may be necessary to support theassembly from the inside. Boeing's preferred support concept isdescribed in U.S. patent application Ser. No. 08/469,985 and involvesusing high temperature silicone rubber balloons or other inflatabledevices to fill the spaces defined by the assembled skins and spars.Other tooling might also be used, such as filling the space with salt oranother material that we can later dissolve, including ceramics. Ifsupport tooling is used, the part design must be such that the supporttooling can be removed after the welds are formed. In this case, forexample, the combination cannot have completely closed cavities. Itgenerally will not because ribs are omitted from the assembledcombination, since welding ribs transverse to the magnetic field istroublesome. The need to remove the support tooling can severely impactthe parts we can fabricate.

The integrity of the weld is critical to the performance of thecompleted, welded structure. The quality of the weld is related to thetemperature along the bond line and good welds require control of thetemperature within a relatively narrow range during the welding. Weparticularly want to avoid overheating, so a "smart" susceptor made froma Co, Ni, or Fe alloy with a Curie temperature slightly above themelting temperature of the resin will help ensure that we producequality welds. By "slightly above" we mean within about 50-70° F. abovethe melting point of Tg for the resin in the processing window for theresin where a weld will form but the resin will not denature or pyrolyzeand the composite will not delaminate. Furthermore, an alloy like Invar42 (42% Ni--58% Fe) has a coefficient of thermal expansion (CTE)comparable to the resin composite so that embedding the susceptor intothe completed part will not have as dramatic an impact if the susceptoris such an alloy rather than copper or another metal where the CTEmismatch between the resin and susceptor is larger.

Suitable thermoplastic resins include polyimides, PEEK, PEK, PEKK, PES,PPS, TORLON (i.e. PEI), or the like. It is especially suited, however,for consolidation or forming of resins that have low volatiles contentand that are nonreactive (i.e., the true thermoplastics like PEEK orULTEM).

The surface of an aircraft wing skin must be maintained to a closetolerance to achieve an efficient aerodynamic surface. The tolerances ofthe inner mold line surface of the wing skin must also be maintained ata close tolerance at least in a buildup area where the wing skin will bejoined to a spar to ensure that the wing skin and spar can be preciselyjoined. It is not as critical, however, to control the inner mold linesurface in areas where the wing skin is not attached to otherstructures. The composite panel has additional plies to define thebuildup areas. The additional reinforcement of the composite panel inthese areas which is necessary where a spar will be attached, andprovide a convenient way to match the skin and spar to produce thedesired outer wing configuration even if the spars are imprecise intheir dimensions. We can fabricate built up areas at the faying surfacesto provide the precision fit, in this way we can eliminate shims.

In conventional thermoplastic welding, the susceptor is a separateelement and may be in sheet, mesh, expanded, milled, selvaged or othersuitable form. The susceptor should be structured for the optimumconductivity longitudinally and transversely needed to obtaincontrolled, reliable, and reproducible heating. Geometry and structureare closely related to the type of induction head used, as those ofordinary skill will understand. In the present invention, we simplyintegrate the conventional susceptor into the detail part along the bondline. Integrated, the susceptor still needs to have all the favorableproperties of the conventional, separate susceptor.

While we prefer to align the longitudinal metal foil strips in thesusceptor with the coil segments, other orientations between the stripsand coil might be used, and such orientations are undoubtedly necessaryto use if we elect to simultaneously weld ribs, spars, and closeouts.

Next, we will discuss the benefits achievable with Z-pins across thebond line, since an integrated susceptor design is compatible withZ-pinning.

3. Z-Pin Reinforcement

Z-pin bonding produces stronger bonds between composite detail partsthan are achievable with adhesive bonds or fusion (i.e., welded) bonds.Z-pin bonding adds Z-direction reinforcement to the otherwiseunreinforced joint in the organic matrix resin. The Z-pins mechanicallyreinforce the bond, especially in environments prone to vibration.Z-pins are beneficial at the joints between detail parts in adhesivebonding, cocuring, or welding processes to join two or more precuredparts or a combination of precured and uncured detail parts.

In the Z-pin bonding process, Pannell prepared a precured composite thathas Z-pins (or "stubble") protruding from the detail along the intendedbond line. To insert the pins in their intended location, he uses aninsertion process like one of those described in U.S. patent applicationSer. No. 08/582,297 or any other suitable insertion process. This basicapproach is shown in FIGS. 14-16. Pannell can also use Avila's pininsertion tool (FIG. 15). Before describing the pin insertion process indetail, we will first describe how to use the Z-pinned detail parts toprepare bonded assemblies.

Throughout this discussion, we use "composite" to mean a fiberreinforced organic resin matrix material. The fibers should be ofsuitable strength to make aerospace structural parts, such as graphite,fiberglass, or carbon fibers. The organic resin can be a thermosettingresin, such as epoxy or bismaleimide, or a thermoplastic, such as ULTEMor KIIIB polyimide. Z-pinning is compatible with all fiber and resinsystems.

The function and properties of the Z-pins are described in copendingU.S. patent applications Ser. Nos. 08/618,650; 08/582,297; 08/658,927;and 08/660,060 which we incorporated by reference earlier. In Z-pinbonding, the resins should be compatible with the intended joint. TheZ-pins might be the same material as the reinforcing fibers in thecomposite detail parts or can be different, as the situation dictates.

Now turning to FIG. 7, the Z-pin bonding process uses a composite detailpart 710 having a region 712 of Z-pin stubble along the intended bondline for connecting part 710 with other detail parts. Each Z-pingenerally protrudes about 1/16 inch above the surface of part 710 (likethe Indian "bed of nails") for ultimate insertion into the facing partsat the joint, but the height can change with the intended application.To protect the stubble during manufacture and inventory prior to layingup the assembly for bonding, we cover the stubble with Teflon peel plys714 and the residue of the pin-carrier foam 716 which we use to hold thepins prior to their insertion into the detail part 710.

In some applications, especially with a decomposable foam, it may beunnecessary to use a peel ply 714. The peel ply 714 functions to protectthe Z-pin stubble during storage while leaving a clean surface in thestubble region when peeled away during the lay up process, and can beany suitable material.

The pins in the stubble 712 may be normal to the surface of part 710 orinterlaced or highly ordered, as described in Boeing's U.S. patentapplication Ser. Nos. 08/618,650 and 08/628,879. That is, the pins canassume any desired arrangement. The density of pins is also variable tosuit the application. Differences in the orientation of pins, theirlength, their strength, their density, etc. can change in differentregions of the bond line. That is, the areal density of pins might be1.0% on the left side of the part in FIG. 7 while being 1.5% on theright side. Alternately, the pin density might be higher aroundfasteners or might be higher near the edges of the bond line as opposedto along the centerline. Also, of course, the orientation may change atdifferent regions along the bond line and orientations might even bemixed together, if desired.

By "orientation," we mean normal, interlaced, highly ordered, or thelike as defined in Boeing's copending Z-pinning applications. Forclarity in the drawings in the present application, we simply illustratethe "normal" orientation.

The pin-carrying foam 716 is described in greater detail in thecopending applications and prior art patents that we earlierincorporated by reference.

The composite detail part 710 can be a laminate of plys of fiberreinforced organic resin matrix material, or might be a sandwich panelwith a foam, honeycomb or other core, or might even be column core.Basically, part 710 can be any material that has a resin interface forbonding to another detail part. We reinforce the joint between the resinand the other part at their interface.

The height to which the stubble of pins extends can vary widely to suitthe intended application. Of course, the strength, shape, size, andorientation of the pins effect their effectiveness when the protrusionheight gets large.

As shown in FIG. 8, the detail part 710a might be fabricated as anI-beam spar rather than as a panel. In fact, the detail part can assumeany shape so long as the shape is susceptible of Z-pin insertion tocreate the reinforcing stubble along the bond line.

FIG. 8 illustrates placing the spar detail part 710a on an uncured panel818 with the stubble 712 at the interface. FIG. 3 illustrates a typicalcross-section of the spar-panel assembly. While the stubble 712 is shownon the spar in FIG. 8, the Z-pins could be on either the spar, the panelor both detail parts.

As shown in FIG. 9, when the assembly of the spar 70a and panel 818 arebonded, the Z-pins in the stubble 712 penetrate into the uncured panel818. In the circumstance where the panel 818a is precured, as shown inFIGS. 10 and 11, we introduce a bond padup strip 820, typically of thesame materials as the detail parts being joined. The padup strip 820 isuncured during assembly and functions to bond the precured, thermosetdetail parts when the bonding process is complete. The padup strip canbe an uncured thermosetting resin prepreg (in which cases bonding is acocuring process) or might be any suitable adhesive bonding material.The padup strip might be a resin encased susceptor of the type shown inFIGS. 19 & 20 and as Boeing uses in its thermoplastic weldingoperations. In this case, the detail parts would generally be precured.

As best shown in FIG. 11, the spar detail part 710a includes a stubblesurface so that the padup strip 820 ends up having pins extendingupwardly from the panel 18a as well as downwardly from the spar flange10a. Shawn Pannell describes in his U.S. patent application Ser. No.08/660,060, that the pins might be carried in the padup strip withstubble on both faces with longer, integral pins if the detail parts arethermoplastic rather than inserting the pins into the spar and panelprior to their curing. We can use Pannell's strips in conjunction withour integrated susceptor detail parts to add Z-pin reinforcement or wecan simply include pins in the Childress design with our detail parts.

FIGS. 12 and 13 illustrate another embodiment of the present inventionwith reference to the bonding of a wing skin to a spar. FIG. 6 shows anexploded view of the wing skin 100, padup strip 20, and spar 200 whileFIG. 7 shows a typical cross-section taken along the bond line. WhileFIGS. 6 & 7 illustrate a wing skinspar joint, the process is applicableto any joint. This embodiment describes bonding using a sandwich corestructure for the wing skin to produce the stubble and subsequentbonding of the skin to the spar with an uncured padup strip in a cocure,adhesive bonding, or welding operation.

As best shown in FIG. 13, the skin 100 comprises a sandwich corestructure of the type described in U.S. patent application Ser. No.08/582,297 having outer face sheets 105 & 110, crushable foam layers 115and 120, and a central foam core 125 with Z-pins 130 extending throughall five layers. Stubble on the interface surface is achieved bycrushing layers 115 and 120 more than the combined thickness of facesheets 105 and 110 during the autoclave cycle when the pins are insertedinto the face sheets. Of course, after curing, the central foam 115, 120and 125 might be dissolved to make a column core skin structure.

The face sheets 105 & 110 are positioned adjacent the foam core 115, 120and 125. We usually use a layer of adhesive to attach adjoining layers.We form the pin-reinforced foam core using known methods (e.g.,stitching or needling) or purchase it from companies such asFoster-Miller, Inc., in Waltham, Mass. We can score the foam coreaccording to the Hoopingarner method to provide channels for venting ofvolatiles during curing.

The core generally is a closed cell foam that includes three sublayers:a high density central sublayer 125 and first and second low density,crushable foam sublayers 115 and 120 located on each side of the highdensity foam sublayer. While three layers are shown, the foam core maybe composed of any number of sublayers depending on the particularapplication. For example, the foam core may be a single low densitysublayer; or, it may be a stack of alternating low density and highdensity sublayers. The foam core should be crushable during autoclavecuring to permit the pins to extend into the face sheets. Low densitypolyimide (e.g., Rohacel™) or low density polystyrene (e.g., Styrofoam™)foams are the presently preferred low density sublayer materials, sincethey are easy to form and do not require extremely high temperatures orpressures to crush during autoclave curing. The low density sublayer maydecompose at the autoclave temperatures.

If a high density sublayer 125 is included, it usually should be made ofa material that will not crush during autoclave curing. Obviously, theprecise temperatures and pressures to be used during autoclave curingwill affect the selection of the material used to form the high densitysublayer. Further considerations to be taken into account when selectingan appropriate high density sublayer material include whether the highdensity sublayer is to be removed after autoclave processing and thepreferred method for removing it. Typically it is high densitypolystyrene or polyimide foam. It might be (i) syntactic foam havinginternal reinforcing spheres, (ii) a fiber-reinforced resin prepreg orcomposite, (iii) a fiberform or microform ceramic such as described inU.S. Pat. Nos. 5,376;598, 5,441,682; and 5,041,321 or in copending U.S.patent applications Ser. Nos. 08/209,847 or 08/460,788, (iv) a metalfoil, (v) a metal foil resin laminate of the type described in U.S. Pat.No. 4,489,123 or U.S. patent application Ser. No. 08/585,304, or (vi) afoam filled honeycomb core. The central sublayer 125 might also be ahoneycomb core with the cells arranged normal to the plane of the facesheets. As Hoopingarner suggests, the core might be a combination ofthese alternatives, like a central honeycomb core bordered by a foamcloseout frame. If the high density sublayer is a prepreg or acomposite, the product itself is a laminated composite. In such case,generally the resin in the face sheets would be the same as the resin inthe high density sublayer.

The Z-pins 130 (here and in all the embodiments) may be any suitablyrigid material, e.g., stainless steel, titanium, copper, graphite,epoxy, composite, glass, carbon, etc. The Young's modules of elasticityfor the Z-pins is generally greater than 10⁷. Additionally, the Z-pinsmay be barbed, where appropriate, to increase their holding strength inthe face sheets.

In the case of thermosets, the face sheets are preferably formed of apartially cured (B-stage) fiber-reinforced composite material. Ifcomposites are used as face sheets, the effect that the autoclave curecycle will have on the face sheets needs to be considered to determineand, then, to follow the optimal temperature/pressure autoclave cureregime.

Suitable reinforcing fibers include glass, graphite, arimide, ceramic,and the like. Suitable resins include epoxy, bismaleimide, polyimide,phenolic, or the like. (Virtually any thermoplastic or thermoset resinwill suffice.)

Various procedures are available for laying up the composite facesheets. Since such procedures are generally known to those skilled inthe arts they are not described here. Although thick, metal sheets donot work well as face sheets, we can use metal foil or metal foil/resinlaminated composites. The metal foil in such cases might be welded tometallic Z-pins in the fashion described in U.S. patent application Ser.No. 08/619,957.

The stubbled skin is bonded to a stubbled spar with a padup strip in theprocess previously described.

FIGS. 14-16 illustrate a preferred process for inserting the Z-pins intoa detail part to leave a stubble interface. The detail part 500 (here alaminated panel having several layers of prepreg) is mounted on a worksurface or layup mandrel 550 with appropriate release films between thepart and tool. Another release film 600 caps the detail part 500 andseparates the part 500 from a Z-pin preform 650 (i.e., a foam loadedwith Z-pins 130 in a predetermined orientation). A rigid caul plate orbacking tool 700 completes the assembly. All the layers are then wrappedin a conventional vacuum bag film 750 which is sealed to permit drawinga suction within the closed volume surrounding the assembly.

As shown in FIG. 15, in an autoclave under elevated temperature andpressure, the foam in the Z-pin preform 650 crushes and the Z-pins 130are driven into the uncured detail part 500. After completing the curecycle, the detail part 500 is cured and has the Z-pins 130 anchoredwithin it. The crushed foam 650 and release ply 600 protect the stubbleuntil assembly of the detail parts is desired. Thus, the process ofFIGS. 14-16 yields a cured detail part having a stubble field. Otherprocesses can be used to achieve the same result, including ultrasonicinsertion into precured thermoplastic laminates as described in theprior art or Boeing's other, copending Z-pin applications.

As shown in FIG. 17, including Z-pin reinforcement in the joint improvescompression after impact strength of the assembly. Boeing tests showabout a 50% increase when using an areal density of 0.5% of 0.006 inchdiameter pins in AS4-3501-6 test specimens following a 20 ft-lb impact.The joint nominally has the same compression and tensile strength priorto impact, but the inclusion of pins increase the compression ultimatestrength when the assembly is subjected to low speed impact energy. Infact, the strain remains essentially constant over the range of impactsless than the impact needed to observe surface damage.

The following examples further illustrate Boeing's Z-pin experiments.

EXAMPLE 1

Childress made 3/16 inch quasi-isotropic composite test specimens fromAS4/3501-6 having 0.5% areal density, 16 mil diameter T300/3501-6 Z-pinswith sufficient surface peel plys to yield 0.080 inch stubble. As acontrol, one-half of the specimens did not insert Z-pins. He assembledtwo of the stubbled parts around an AS4/3501-6 uncured scrim padup about0.090 inch thick with the stubble from each part overlapping. He bondedthe assembled parts using the conventional bonding cycle. Then, he cutthe resulting bonded assembly into 1×10 inch coupons, thereby havingsome pin-reinforced, bonded coupons and some coupons lacking pinreinforcement.

Boeing tested the specimens in the G_(lc) Mode 1 fatigue test cycle withpull tabs glued to the faces pulled in a standard G_(lc) test fixture,and included a crack starter initiating peel in the bond area. Thecorrelated data with the standard load v. head extension (in-lbs/in) issummarized in Table 1:

                  TABLE 1    ______________________________________    Specimen   Load   Comments    ______________________________________    Pinned: 1      5.4            2      4.8            3      2.86   *Failed in the laminate above the bond line    Unpinned:            4      2.75            5      3.64            6      3.49    ______________________________________

Ignoring specimen 3, the Z-pin reinforcement at this relatively lowdensity improved the bond strength with this Mode 1 fatigue measure byabout a 45% increase in the peel strength. Upon analysis of the pinnedspecimens, Childress discovered that some pins were bent, which hebelieves lowered the reinforcing value (reduced the measured load).Boeing also believes that it could prepare even better bonds (i.e.joints) using higher pressure during the bonding cycle.

EXAMPLE 2

Childress prepared additional specimens using AS4/3501-6 prepreg with 2%by area 0.020 diameter titanium Z-pins inserted into a spar cap. Hecured the spar at 350° F. with Z-pin stubble left exposed on the sparcap. The Z-pin stubble was 0.20 inches long. This cured spar was thenplaced on an uncured skin laminate 0.30 inches thick, with the Z-pinstubble placed against the uncured skin. The spar, associated spartooling, and skin were then vacuum bagged and autoclave cured at 350° F.using a 100 psig autoclave pressure. The vacuum and autoclave pressuredrove the spar down onto the uncured skin and inserted the Z-pin stubbleinto the skin. The cured final part was then trimmed for pull testing.

Pull testing results showed the Z-bonded parts had an 83 percent greaterload carrying capability than the control parts. The results aresummarized in FIG. 18.

In a thermoplastic welding process, the padup strip 820 might include asusceptor for integrating with an oscillating magnetic field to generateeddy currents sufficient to melt and cure the bond line resins and toform a weld. This strip could have the susceptor adjacent one surface sothat the strip would be the analog of one of our integrated susceptordetail part, albeit a thin one. Generally, however, the susceptor wouldbe centrally located in the strip in which case there would be littlebenefit from the susceptor's in our detail parts if the method ofheating involved induction. Of course, if the part carried an integralsusceptor and the strip included a susceptor, for welding in the fixedcoil induction heating press, a loop circuit could be fashioned byadding jumpers between the part and strip. Also, the multiple susceptorsmight be suited for resistance heating. Boeing prefers to reduce theamount of metal at the bond line so we prefer a single susceptor design.

We can use any other arrangement to get the appropriate heating at thebond line when completing the weld. If welding, we prefer to use pins inthe detail parts that penetrate further into the parts than the regionwhich softens during the formation of a fusion bond between the details.In this way, the pins stay firmly anchored in their desired orientation.A suitable padup strip is illustrated in FIG. 19. We can heat the bondline with induction or resistance heating produced in susceptor. Any ofBoeing's susceptors might be used. Energy can be introduced for heatingto the susceptor by induction, resistance, a combination of both, or anyother suitable means.

As shown in FIG. 19, the susceptor 1300 typically comprises a metal mesh1305 encased in a resin 1310. The susceptor of FIG. 19 includes selvageedge strips 1315. The mesh 1305 includes a repeating pattern of diamondshaped openings of length (L) and width (W) separated by fine-lineelements.

FIGS. 21-23 illustrate Avila's pin insertion tool that we can use toform detail parts having pin stubble. Avila's tool 1500 incudes ahousing 1505 holding a sliding piston 1510 which is reciprocal between aloading position for receiving a pin-carrying foam 1550 in a cavity 1515and an insertion position where the piston moves upwardly to crush thefoam and to insert the pins 1545. Seals 1520 permit the piston 1510 toslide along the walls of housing 1505 when pneumatic pressure is appliedthrough inlet 1525 to chamber 1530 behind the piston. Motion of thepiston 1510 toward removable cure tool 1535 is arrested with stop 1540which also serves to control the depth of insertion of pins 1545 in thepin-carrying foam 1550 into the detail part 1555. The stop 1540 contactsreplaceable stop 1560 that seats in the fixed support frame of the curetool 1535 that is rigidly attached to the housing 1505 as the fixed walldefining cavity 1515. The replaceable stop allows adjustment of thedepth of penetration of the pins into the detail part 1555. The curetool 1535 fits rigidly in a matching receiving surface in the frame anddoes not move when piston 1510 moves upwardly. Yet, cure tool 1535 isreplaceable to permit controlled insertion of different Z-pinorientations or different insertion depths into the detail part 1555.During pin insertion through movement of the piston 1510, the detailpart 1555 is held rigidly on the surface of the cure tool 1535 so thatthe Z-pins 1545 are positioned correctly.

All parts of the pin insertion tool 1500 are designed to withstand thetemperatures and pressures associated with autoclave curing of the resincomposite detail parts. Any necessary release films can be used betweenthe pin-carrying foam 1550 or the detail part 1555 and the working partsof Avila's insertion tool.

As the piston 1510 moves upwardly to compress the pin-carrying foam 1550against the cure tool 1535, the Z-pins 1545 in the foam register with anassociated hole 1605 (FIGS. 16 or 17) in the cure tool 1535. To assureregistration between the pin 1545 and hole 1605, each hole has a funnelnozzle 1705 to guide the pin into the hole and into its properorientation in the detail part.

The cure tool has the arrangement of holes that corresponds with thedesired Z-pin orientation in the detail part. The tool helps placing thepins accurately. Because the foam decomposes at the autoclave curingtemperature, without Avila's tool, the pins lose their lateral supportand can move or buckle to disturb the desired pin orientation,especially when the stubble field in the detail part covers a largearea. For further assurance of proper pin placement, the contact face ofpiston 1510 might be knurled to keep the pins from sliding.

Avila's tool might include a shearing ram on the contact surface betweenthe cure tool and the detail part or at the interface between the curetool and the pin-carrying foam for cutting the pins after theirinsertion. In the alternative where the ram is at the cure tool-foaminterface, the width of the cure tool becomes a reliable gauge forsetting the height of the stubble, since this much of the pins willprotrude when the detail part is removed from the tool.

Avila's pin insertion tool is especially beneficial when makingrelatively large production runs of detail parts. The tool reducespart-to-part variation by inserting the Z-pins accurately and repeatedlywhere they are designed to be. Avila's tool is described in greaterdetail in U.S. patent application Ser. No. 08/657,859, which weincorporated by reference.

For combining the integrated susceptor and Z-pins we might simply applya metal foil which the pins pierce on insertion to yield a patternedsusceptor. We suspect, however, that it would be difficult to obtaincontrolled heating of this pieced susceptor reliably from part to partand configuration to configuration. Therefore, we prefer to position thepins in the pre-existing openings of the expanded foil. The susceptormight be fashioned as Kirkwood et al. suggested in U.S. patentapplication Ser. No. 08/486,560 as a "barbed wire" analog.Alternatively, the susceptor might be of the reinforced design woven andaligned with the associated fiber in the reinforcing fabric. Then, thefibers will protect the delicate foil when the pins are inserted.

4. An Integrated Susceptor

Now turning to the integrated susceptor of the present invention asshown in FIG. 25, we bond the susceptor 2405 to the detail part 2410along the bond line using an intermediate thermoplastic film 2415 toproduce a detail part analogous to the Childress Z-pinned detail part.In fact, in a single step, we can integrate the susceptor and Z-pin thedetail part. In this case, the stubble field would overlay the susceptor2405.

The susceptor typically is a 5 mil thick copper foil, but it may betailored, selvaged, "smart," reinforced, structural, or any othersusceptor configuration described in Boeing's patents and patentapplications. The intended application for the detail part and itsmethod of final assembly factor into the decision on the appropriatesusceptor. While our focus is primarily with induction thermoplasticwelding, the susceptor can be heated resistively or by any othereffective method. While FIG. 24 shows the susceptor 2405 over the entiresurface of the part, typically the susceptor will be adhered in only asmall portion of the surface corresponding essentially to the area ofthe bond lines. If the entire surface is covered, of course, definitionof the bond line can wait until the detail parts are assembled, butprecise identification of the bond line with a narrow susceptor is theapproach we prefer. Placing the susceptor at a fixed location on thefaying surface of the detail makes it easier to accurately locate thesusceptor at the bond line interface because the susceptor is tied toone of the interfacing detail parts. Accurate placement is important,especially for the induction process, because the time-varying magneticfield that the coils generate induces uneven heating if the coil andsusceptor are misaligned. This problem of alignment is discussed ingreater detail in U.S. patent application Ser. No. 08/564,566, which weincorporate by reference. Also, integrating the susceptor with thedetail part protects the delicate foil against damage, helps us toassemble complex curvatures, and lends itself to process automation byreducing part count and positioning dexterity.

We can prepare the integrated susceptor while laying up the detail partin fiber placement or prepreg manufacturing operations or can add thesusceptor to a prefabricated detail part. When working with prepregs, atthe intended location of the bond line, we add a thermoplastic film tothe prepreg surface to make the surface resin rich. Then, we overlay thesusceptor on the thermoplastic film. We consolidate and cure the preformin conventional autoclave operations to prefabricate the integratedsusceptor detail part. In resin transfer molding, we position the filmand susceptor or simply the susceptor in the appropriate place in themold before injecting the resin around the fiber preform.

For prefabricated detail parts, we prepare the surface of the part inthe intended location of the susceptor with an alcohol wipe. Then, weposition the thermoplastic film 2415 and susceptor 2405 in the samemanner as we did for the prepreg manufacture. We tack the film andsusceptor in place with KAPTON high temperature tape and place thepreform in a heated oven to melt the thermoplastic film. When thepreform is heated appropriately, we remove the hot part from the ovenand press the susceptor to consolidate the susceptor and thermoplasticfilm with the prefabricated detail.

The film 2415 assures us of a resin rich surface at the weld. We mightadd reinforcing fiber to the film or over the film or in the bond lineto alleviate residual tensile strain in the weld.

EXAMPLE 3

We formed a 10 ply thick test panel from PEEK (APC-2/AS4) graphiteunidirectional tape. After preparing the surface of the entire 9×15 inchpanel, we applied 10 mils of PEEK adhesive film and a 10 mil thick coldrolled steel susceptor of conventional design. We heated the preform toabout 780-800° F. in an oven and removed the hot preform to a presswhere we applied a consolidation pressure of about 1500 psi for 5minutes until the panel had cooled below its glass transitiontemperature (Tg).

EXAMPLE 4

We formed second and third test panels using 5 mil copper mesh and PEEK(APC-HTA/IM8) prepreg material according to the process of Example 3.For the prepreg material we reduced the oven temperature to 650-680° F.

Upon removal from the press the integrated susceptor detail parts aretrimmed to net size and are used in subsequent thermoplastic welding(fusion bond) operations as we have described. We can add Z-pins to thedetail part if the consolidation press is configured like Avila's pininsertion tool and if we place the susceptor on the cure tool (guidebusing) for directing the proper placement of the pins.

Our welding induction coil 2500 and process generally are shown in FIG.25 with respect to the formation of a lap shear joint between twothermoplastic APC-2/AS4 composites sheets 2505 and 2510. The susceptor2405 is integrated onto the edge of lower composite 2505 on fayingsurface defining the bond line (i.e. where the two composites overlap).The coil 2500 is a transverse flux coil having a primary coil 2515 andsecondary coil 2520. The secondary coil 2520 preheats the bond linewhile the primary coil 2515 actually provides the heating necessary tocomplete the weld. The coil 2500 moves over the bond line at a rate ofabout 4-5 inch/min applying a bond line pressure of about 20-450 psi,and, preferably, 20-60 psi through leading roller 2605 (FIG. 26) andtrailing rollers 2610. Force is exerted on the traveling coil 2500through column 2615 and carriage 2620 that is attached to a gantrysystem (not shown) overlying the composite assembly.

The part geometry generally is without contour or with simple contour ata relatively large radius of curvature (for example, about 115 inches).The composites are about 0.100-0.250 inches thick. The bond line isabout two inches wide. The susceptor is copper foil about 5 mils thickpatterned to achieve substantially uniform heating on the bond line.With these parameters and suitable backup phendic tooling to retain theassembly geometry, we have achieved joints having an average strength ofabout 7000±350 psi.

As with our cup coil and asymmetric coil, we cool the coils 2515 and2520 during operation, introducing cooling water through a plumbingcircuit that includes inlet coupling 2525, conduit 2530, distributor2535, outlet line 2540, and outlet coupling 2545. If the plumbingcircuit is copper tubing, we can input power to the coils 2515 and 2520through the circuit from a 50 kW-50 kHz solid state power supply coupledwith a 50 kHz-50 kW isolation transformer to provide low frequency(around 8 kHz) power to the coil 2500. While we have attempted weldingat frequencies between 95-200 kHz, we have discovered that in this highfrequency range the magnetic field couples with the reinforcing fibersand induces eddy currents in the fibers. Such coupling leads tooverheating of the assembly and delamination of the composites. At lowfrequencies, the energy is focused on the susceptor at the bond line inthe vicinity of the weld without any significant heating in thereinforcing fibers.

We investigated process alternatives using a preheating step as well asa post-weld anneal to control the cooling of the bond line. FIG. 29graphs the improvement we measured. Preheating tests were conducted at200° F. and 300° F., respectively. Annealing involved retaining theassembly at 500° F. for 20 minutes. We conducted the heating in an ovenat atmospheric pressure.

We discovered that there was little advantage with preheating, but thatthe post-weld anneal and its controlled cooldown offered a significantstrength improvement and a higher quality weld. Parts made with theanneal also showed lower process variance.

FIGS. 26-28 illustrate more realistic aerospace structural assembliesthat we have welded. In particular these figures illustrate a skin/sparjoint wherein a "C" spar 2705 is welded to skin panels 2710 and 2715through a series of lap shear joints. As shown in FIG. 26, the compositeparts are pinned together (area 2650) in the trim region of the detailparts to define the bond line 2675 where the susceptor rests between thefaying surfaces. The coil 2500 in FIG. 26 travels from left to rightover the bond line 2675 to create the fusion bond or weld 2750 (FIG.27). As best shown in FIG. 27, the weld 2750 can interconnect skin panel2710 to spar flange 2720 and skin shoulder 2725 or the skin panel 2710might be welded to the shoulder through weld 2750a, and the shoulderseparately welded to the spar flange 2720 through weld 2750b. In thecase of the single weld 2750, a tongue 2730 on skin panel 2715 might bewelded, bonded, fastened, or otherwise connected to the web of spar 2705to provide the desired strength and rigidity in the structure.

In an alternate construction, a shoulder 2800 on the end of skin panel(not shown). In this arrangement or the corresponding one illustrated atthe bottom of FIG. 27, Z-pins through the aligned faying surfaces wouldbe particularly beneficial and relatively easy to introduce to thebonded assembly.

While we have described preferred embodiments, those skilled in the artwill readily recognize alterations, variations, and modifications whichmight be made without departing from the inventive concept. Therefore,interpret the claims liberally with the support of the full range ofequivalents known to those of ordinary skill based upon thisdescription. The examples illustrate the invention and are not intendedto limit it. Accordingly, define the invention with the claims and limitthe claims only as necessary in view of the pertinent prior art.

We claim:
 1. An unitary prefabricated resin composite structure forjoining the structure to another composite detail part with a fusionbond, comprising:(a) one or more plies of a fiber reinforced resindefining a faying surface; (b) a thermoplastic film as a surface plyadhered on the faying surface to define a resin rich bond line; and (c)an integrated metal mesh susceptor bonded to the film at the bond line,the susceptor being heatable under the influence of an oscillatingmagnetic field to a temperature sufficient to melt the resin and thefilm to form a fusion bond between the structure and another partadjacent the faying surface at the bond line.
 2. An unitaryprefabricated resin composite structure for joining the structure toanother composite detail part with a fusion bond, comprising;(a) a resincomposite detail part including at least one ply of a fiber reinforcedresin, the part having a faying surface; (b) a thermoplastic filmadhered to the part on the faying surface to define a resin rich bondline; (c) an integrated metal mesh susceptor bonded to the film at thebond line, the susceptor being heatable under the influence of anoscillating magnetic field to a temperature sufficient to melt the resinand the film to form a fusion bond between the structure and anotherpart adjacent the faying surface at the bond line; and (d) Z-pinreinforcement stubble along the bond line and interleaved with thesusceptor.
 3. The structure of claim 2 wherein the thermoplastic filmand the resin are the same composition.
 4. The structure of claim 2wherein the film is less than about 10 mils thick and the susceptor isless than 10 mils thick.
 5. The structure of claim 2 being an aerospacespar having a web and a cap connected to the web, the susceptor beingadhered to the cap.
 6. The detail part of claim 1 wherein the susceptoris copper.
 7. The detail part of claim 2 wherein the susceptor is ametal mesh.
 8. The detail part of claim 7 wherein the metal is copper.9. The detail part of claim 2 wherein the stubble is present at an arealdensity of about 0.375-1.50%.
 10. The detail part of claim 9 wherein thestubble includes chopped carbon fibers.